Muscle atrophy is a prominent and debilitating medical condition in humans, but the signaling molecules that initiate this process have not been elucidated yet. Our goal is to identify novel molecule(s) that regulate pathways involved in protein synthesis and/or protein degradation, leading to regulation of muscle mass. We hypothesize in this study that TRB3 mediates denervation-induced skeletal muscle atrophy and that inhibition of TRB3 ameliorates the loss of muscle mass. This hypothesis has been formulated on the basis of preliminary data showing that TRB3 expression is induced by muscle denervation and that overexpression of TRB3 inhibits Akt activity. Furthermore, knockout of TRB3 in mice results in increased Akt activity and decreased FOXO function. We will test the hypothesis by pursing two specific aims. In Aim 1, we will determine TRB3 expression and function in response to skeletal muscle denervation. Under this aim, we will overexpress TRB3 in skeletal muscle using electroporation technique and muscle-specific TRB3 transgenic mice, and assess the denervation-induced muscle atrophy. Guided by our preliminary data, we anticipate that overexpression of TRB3 in muscle will worsen muscle atrophy. In Aim 2, we will determine if TRB3 inhibition prevents denervation-induced skeletal muscle atrophy. To this end, we will study TRB3 knockout mice to determine if the mice are resistant to denervation-induced muscle mass loss. We will also determine protein synthesis and Akt activity in the knockout mice. We expect that knockout of TRB3 will prevent mice from developing denervation-induced muscle atrophy through ablation of decreases in protein synthesis and Akt activity in response to muscle denervation. The proposed research will establish the novel function of TRB3 on muscle atrophy and identify a new approach for treating muscle atrophy. In addition, the study will provide evidence to investigate the role of TRB3 in other muscle diseases related to muscle atrophy, including muscle disuse, cachexia, and sarcopenia. The study will be innovative in 1) proposing new insight into the cellular and molecular etiology of muscle atrophy development; 2) studying unique animals that model both Gain-of- function (TRB3 transgenic mice) and Loss-of-function (TRB3 knockout mice); and 3) investigating a novel function of TRB3 on muscle mass regulation. The proposed study will be also significant in providing new insights into the pathogenesis of muscle atrophy and in holding promise for the development of pharmacological agents that could prevent muscle atrophy during a wide range of diseased states. Furthermore, the knowledge that we will gain from this project can be applied to other muscle diseases that are directly or indirectly related to muscle atrophy.